This invention relates to a drive system for a vehicle, and in particular to an auxiliary drive system for driving the normally non-powered front wheels thereof.
Vehicles, are typically driven by applying power to the rear wheels thereof. It has been found that productivity of the vehicle can be increased by selectively applying an auxiliary drive system to the normally non-powered front wheels. The application of power to the front wheels is particularly effective where the tractive conditions are poor or too variable, as when operating in snow, mud, sand or ice. Although, sometimes such adverse conditions exist for only short periods of time, adequate traction is needed to maintain the vehicle's productivity at a reasonable level and to prevent the vehicle from becoming stuck.
When the vehicle is a construction machine such as a motor grader, powering the front wheels has the additional benefit of helping to counteract the side force resulting from angling the mold board during grading operations. Although wheel lean and additional front axle weight help, front wheel drive increases productivity in such cases.
A number of physical characteristics effect the application of a front wheel drive to a vehicle, such as a construction machine, particularly when the machine is a motor grader or similar type of apparatus. For example, the motor grader has a long wheel base and generally has the blade and its support mechanism located between the rear wheel drive axle and the front axle. The machine also has high steering angles and the front wheels must be capable of leaning for various applications.
The utilization of a hydrostatic drive as the front wheel assist drive resolves many of the problems discussed above. However, the utilization of a hydrostatic drive also generates other problems, as for example, conventional hydrostatic systems inherently yield dynamic braking which may be an undesirable characteristic. Further, the synchronization of the speed of a hydrostatic drive to a gear or power shift drive normally used to power the rear wheels is difficult.